The reverse reaction of esterification is called ester cleavage or ester hydrolysis. In this hydrolysis, one mole of water is consumed per mole of ester bond, giving rise to one mole each of free acid and alcohol. Being the reverse reaction of esterification, hydrolysis is likewise an equilibrium reaction.
In oleo-technology, the hydrolysis of triglycerides, i.e. the hydrolysis of oils and fats of vegetable and animal origin, is a procedure well known to those skilled in the art for preparing free fatty acids. For instance, triglycerides are hydrolysed with addition and consumption of water at temperatures of 200° C. or higher and at a corresponding water vapour pressure in the liquid phase to glycerol and free fatty acids (FFA). An example of an industrial embodiment of this process is the Lurgi splitting tower process. This type of reaction regime for ester hydrolysis is established in industry and proceeds with high efficiency, since the glycerol that forms separates out of the reaction mixture as a separate phase during the reaction and hence promotes a shift in the reaction equilibrium in the direction of the FFA target reaction product. Further details of the known procedures for hydrolysis of triglycerides can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, under “Fatty Acids”, chapter 3.2 “Fat Splitting”.
For preparation of fatty acids by hydrolysis of fatty acid alkyl esters, especially of fatty acid methyl esters (FAME), the literature describes processes which counteract establishment of equilibrium by evaporation of methanol formed out of the reaction mixture. These processes work at low pressure, for example ambient pressure, within a temperature range of, for example, 70 to 150° C. As a result of these low reaction temperatures, it is necessary to catalytically accelerate the reaction in order to achieve the desired high conversions based on industry standard reaction times and residence times.
For example, patent publication DE 69321607 T2 describes splitting of a FAME mixture of methyl caprylate and methyl capronate, conducted at ambient pressure in the range from 70 to 110° C., wherein an acidic, homogeneously dissolved catalyst comprising alkylbenzenesulphonic acids is used. As is the case with many homogeneously catalysed processes, here too, there is the drawback of separation and workup for reuse of the catalyst from the reaction mixture. A distillative workup of the reaction mixture under reduced pressure is likewise described here, wherein, in a first stage, methanol, water and unconverted fatty acid methyl ester are removed. In a second stage, the FFA product is then separated from the catalyst and the latter is recycled into the reaction system.
US patent specification U.S. Pat. No. 4,185,027 describes an acid-catalysed process using sulphuric acid, p-toluenesulphonic acid or acidic ion exchanger within a similar temperature range to that in DE 69321607 T2, wherein propionic acid is additionally added as short-chain carboxylic acid. This reacts with release of the fatty acid to give methyl propionate as an intermediate. In this case too, the short-chain carboxylic acid added, as well as the catalyst, has to be separated from the reaction mixture in a costly and inconvenient manner. In the case of use of ion exchangers as catalyst, the removal of catalyst is simplified, but the conversions described are much lower compared to the conversions achieved in the case of homogeneous catalysis (sulphuric acid, p-toluenesulphonic acid), or high concentrations of, for example, 12 to 27 g of ion exchanger are required per 100 g of FAME in order to achieve high conversions within an appropriate time. Moreover, in this variant too, the propionic acid added finally has to be removed from the reaction mixture.